Page 42 - Reservoir Geomechanics

26 Reservoir geomechanics

materials are so extremely weak that there can be essentially no shear stress acting
on the interface between the salt (or overpressured shale) and the adjacent formations.
This means that there will be a tendency for principal stresses to re-orient themselves
to be parallel and perpendicular to these weak planes. Yassir and Bell (1994)argue
that maximum horizontal stress orientations on the Scotian shelf are controlled by the
sloping interface of severely overpressured shale at depth. Yassir and Zerwer (1997)
show that stress orientations in the Gulf of Mexico are locally affected by salt bodies.
It is clear that ignoring how stress orientation and magnitude are affected by the
presence of salt bodies can lead to costly well failures. In the cases illustrated in
Figure 1.10, wells were planned that would be influenced by the modified state of stress
around the salt body. Because principal stresses must align parallel and perpendicular
to the salt interface, the lines in Figure 1.10b show schematically how the maximum
and minimum principal stresses might be deflected by the presence of the salt. In this
case the well trajectory tracks beneath the salt body. Needless to say, the principal
stresses along the well trajectory deviate markedly from horizontal and vertical and
this must be taken into account when considering the stability of such a well. In two of
the cases illustrated in Figure 1.10a, the well trajectories involve drilling through the
salt such that markedly different stress fields are expected above, within and below the
salt bodies.
A theoretical analysis of idealized salt bodies at depth has been considered by
Fredrich, Coblentz et al.(2003) who used non-linear finite element modeling to illus-
trate the variation of stress around and within the bodies. Such calculations, when used
in accordance with the principles of the stability of deviated wells (as discussed in
Chapters 8 and 10) could be quite effective in preventing costly well failures in areas
of particularly complicated in situ stress. Some of the more interesting findings of the
Fredrich, Coblentz et al.(2003) study are that stresses within the salt body may not
be uniformly equal to lithostatic (as commonly assumed) and that some of the drilling
problems encountered when drilling through the bottom of a salt structure that are
usually attributed to very weak rock in a hypothesized rubble-zone, may actually be
associated with concentrated stresses at the bottom of the salt body.